Cavity resonators, have been conventionally used in electromagnetic heaters for heating living bodies and objects. According to a typical electromagnetic heating device, as shown in FIGS. 33(A) and 33(B), a pair of projections (reentrants) 4a and 4b are integrally provided in the upper and lower internal walls 2a and 2b of a cavity resonator 1 consisting of a cylindrical metallic conductor, and a pair of openings 3a and 3b are provided in its side wall 6 to introduce a living body 5 or an object thereinto. High frequency electromagnetic energy is supplied to the cavity resonator 1 from outside.
In such an electromagnetic heating device, it is desired to be able to concentrate high frequency electromagnetic energy to a desired spot at a high efficiency. However, it is known that when the size of the object 5 to be heated changes, the resonant frequency of the cavity resonator changes to such an extent that an efficient electromagnetic heating effect cannot be attained. Therefore, it was necessary to change the frequency of the oscillator for supplying high frequency energy according to the size of the object in order to achieve an appropriate heating. However, when the oscillation frequency of a high frequency oscillator is to be made variable, the ocisllator becomes accordingly more complex, and combined with the need to achieve an impedance matching anew every time a change in the frequency occurs the device would be even more complex and expensive.
It is conceivable to provide a link coil as means for adjusting the resonant frequency of the cavity resonator and achieving an efficient transmission of high frequency energy into the cavity resonator. However, the use of a link coil does not necessarily ensure an efficient supply of high frequency energy cavity resonator produces a change in the impedance of the cavity resonator as seen from the high frequency electromagnetic energy source.
Further, it is known that the point of concentration of electric current is difficult to keep fixed, and that it tends to shift according to the shape and electric constants of the object to be heated. For this reason, it has been impossible to keep a location to be heated at a high temperature all the time. It has therefore been desired to be able to concentrate high frequency energy to a highly localized region of the object to be heated irrespective of the conditions of the object to be heated.